python：用户站点中的包不覆盖ubuntu上的dist-packages

$pip install --user --upgrade six

这正确地在我的主目录中安装了六个最新版本(1.4.1).但是,当我加载刚刚安装的软件包时,python选择在系统范围内安装的版本：

$python -c "import six; print six; print six.__version__"                                                     
<module 'six' from '/usr/lib/python2.7/dist-packages/six.pyc'>
1.2.0

这在我看来是奇怪的行为;应首选用户手动安装的软件包.我在Arch Linux上尝试过相同的过程,首先加载用户站点包.我的sys.path：

$python -c "import sys; print sys.path"                                                                       
['','/home/jostein/.local/lib/python2.7/site-packages/enaml-0.5.1-py2.7.egg','/usr/lib/python2.7/dist-packages','/home/jostein/Programming/scikit-image','/home/jostein/Programming/reconstruct','/home/jostein/Programming/ptychography','/home/jostein/Programming','/home/jostein','/usr/lib/python2.7','/usr/lib/python2.7/plat-x86_64-linux-gnu','/usr/lib/python2.7/lib-tk','/usr/lib/python2.7/lib-old','/usr/lib/python2.7/lib-dynload','/home/jostein/.local/lib/python2.7/site-packages','/usr/local/lib/python2.7/dist-packages','/usr/lib/python2.7/dist-packages/PILcompat','/usr/lib/python2.7/dist-packages/gst-0.10','/usr/lib/python2.7/dist-packages/gtk-2.0','/usr/lib/pymodules/python2.7','/usr/lib/python2.7/dist-packages/ubuntu-sso-client','/usr/lib/python2.7/dist-packages/ubuntuone-client','/usr/lib/python2.7/dist-packages/ubuntuone-control-panel','/usr/lib/python2.7/dist-packages/ubuntuone-couch','/usr/lib/python2.7/dist-packages/ubuntuone-storage-protocol','/usr/lib/python2.7/dist-packages/wx-2.8-gtk2-unicode']

/usr/lib/python2.7/dist-packages出现在与用户相关的任何内容之前,这可能是导致问题的原因.

有没有人知道在ubuntu上允许用户安装的软件包优于系统级安装的好方法？